Optical bodies with non-epitaxially grown crystals on surface

ABSTRACT

This is a process for growing non-epitaxially a different crystal upon a base crystal surface and thereafter destroying said different crystal, thereby generating a uniformly random arrangement of surface irregularities which are independent of lattice orientation in the base crystal. The texture of the treated surface is unlike prior art surfaces, whether produced chemically or mechanically, and specular reflections are essentially eliminated.

United States Patent 191 Swinehart et al.

[ Sept. 24, 1974 OPTICAL BODIES WITH NON-EPITAXIALLY GROWN CRYSTALS ONSURFACE [75] Inventors: Carl F. Swinehart, University Heights; James T.Lindow, Cleveland Heights, both of Ohio [73] Assignee: Kewanee OilCompany, Bryn Mawr,

22 Filedz Sept. 2, 1971 [21] Appl.No.: 177,447

[52] US. Cl 117/16, 117/47 R, 117/118, 156/2, 156/5, 156/7, 252/301.4 H[51] Int. Cl B44d 5/02, C09k 1/06 [58] Field of Search 117/47 R, 118,16; 156/2, 156/4, 5, 7; 250/71.5 R; 252/3014 H [56] References CitedUNITED STATES PATENTS 1,482,793 2/1924 Hartmann 156/2 2,666,145 6/1949Eversole et al. 250/71.5 R 2,822,250 2/1958 de Nobel 156/17 UX 3,068,35911/1962 Carlson 260/751 R 3,089,793 5/1963 Jordan et al. 156/17 UX3,102,955 9/1963 Carlson 250/75.l R 3,235,426 2/1966 Bruner 156/23,483,048 11/1969 Lenz et al. 156/2 3,490,982 1/1970 Saveniere et al.117/118 X 3,595,718 7/1971 Fishman et al .1 156/2 FOREIGN PATENTS ORAPPLICATIONS 757,814 9/1956 Great Britain 156/2 Primary ExaminerWilliamD. Martin Assistant ExaminerWilliam H. Schmidt Attorney, Agent, orFirm-Cain and Lobo [57] ABSTRACT 14 Claims, N0 Drawings OPTICAL BODIESWITH NON-EPITAXIALLY GROWN CRYSTALS ON SURFACE BACKGROUND OF THEINVENTION Crystal growth has a history of interest with respect to theepitaxial growth of crystals and much work has been done in this regard.The instant invention is oppositely directed to a process for producinggrowth on a crystal non-epitaxially. By epitaxy is generally meant thatthe growth structure is determined by the crystal structure and orientedby the underlying substrate. The dictionary defines epitaxy as theoriented overgrowth of one crystalline substance on a substrate ofdifferent chemical composition. In a strict crystallographic sense,epitaxy is confined to the oriented growth of a crystalline substance inthe same lattice configurations as a continuation of the structure ofthe base crystal. In whatever sense it is considered, the instantinvention is concerned with the disoriented growth of a different anddestroyable form of the base crystal capable of generating uniformlyrandom surface irregularities. More particularly, it is concerned withnon-epitaxial growth on preselected surfaces of a scintillation crystal,which is a necessary intermediate step in the improvement of itsresolution.

In general, scintillation crystals, also known as scintillationphosphors or scintillators are classified in four categories as used foralpha, beta, and gamma-ray detection: inorganic crystals, organiccrystals, plastic phosphors, and liquid phosphors. Typical organicscintillation phosphors are thallium-activated sodium io' dide,europium-activated lithium iodide, thalliumactivated cesium iodide,sodium-activated cesium iodide, and thallium-activated potassium iodide;typical organic scintillation phosphors are anthracene andtrans-stilbene crystals. The instant invention is concerned mostparticularly with inorganic scintillators. The primary advantage of theinorganic crystals over the other types is their higher density, whichis mainly responsible for the higher stopping power and thus the greatercounting efficiency for gamma-rays. Since the alkali halides, and inparticular thallium-activated sodium iodide, exhibit such desirablecharacteristics as high density, high light output, and transparency, itis this material which is normally used for camera plates andspectrometer units, which require excellent resolution.

It has been observed that the resolution of a scintillator having adiffuse surface is superior to one having a polished surface. This istrue both for plastic and crystalline scintillators, the effectincreasing as the index of refraction increases and being mostpronounced for symmetrical shapes. (Applied Gamma-Ray Spectrometry, p.57, edited by C. E. Crouthamel, Pergamon Press 1960). Diffuse-reflectionof light at the surface of a scintillation phosphor is of criticalimportance in the performance of crystal scintillation detectors. Toimprove resolution of a crystal and compensate for geometrical shape,its surfaces are usually sanded, scratched, ground, or polished invarious areas in particular ways, mostly according to unwritten ruleslearned by trial over an extended period of time. The results of suchmechanical treatment are judged by experimental measurement of theenergy resolution of the crystal as a gamma-ray spectrometer.

Theoretically, the best resolutions, i.e., the smallest ratio of thewidth of the peak at half maximum to the energy of the peaks midpoint,should be obtained if all the light from each event in a scintillationcrystal reached the sensing surface of a light detector and was absorbedby it. This could only result if there were no light absorption in thecrystal and one hundred per cent reflection at all surfaces except wherethe sensing element is attached. Also the sensor should respond equallyto light across its surface and for any angle of incidence. Even in thindiscs, the foregoing conditions cannot be realized in actual detectors.A second approach to obtain optimum resolution is equal sampling oflight from events in all parts of the crystal, assuming the electroniccircuitry is able to accommodate the weakened signal. Since it isgenerally accepted that none of the above theoretical conditions can bemet in practice, compromise techniques must be used depending upon thegeometry of the crystal. For example, it may be desirable to roughensome areas more than others, even resorting to polishing some areas tolet light out, as described in more detail in US. Pat. No. 3,102,955.

Modern scintillation scanning devices are disclosed in US. Pat. No. Re.26,014 and in US. Pat. No. 3,159,744, in which scanners are described indetail, along with the circuitry required. Generally speaking, thescintillation detector is a device which is used in medical science tomonitor the level of the radiation emanating from a patient suspected ofhaving a tumor, cancerous condition, or other physical problem afteradministering a radioactive tracer. The patient is positioned below ascintillation detector and the energy emanating from the patient isdetected by it. Energy detected by the scintillation detector or probeis converted to light and then transformed by phototubes to electricalsignals, which are then selectively conducted to a suitable pulse heightanalyzer which stores and sorts the data. Depending upon the particularcircuitry of the measuring device, an image of the intensitydistribution emanating from the patient may be obtained. Such images arecomposed of dots, which vary in concentration according to the activitydetected so that the physician is able to determine not only the area inwhich the activity is present but its distribution.

Isotopes currently used in the biomedical field are in the below-350 KEVrange; typically, KEV from Technitium 99. At this relatively low energylevel the camera image quality is limited by the photon statistics ofthe relatively few photons produced by a scintillation crystal. Imageresolution improves inversely as the square root of the number ofphotons.

The degree of surface roughening of a scintillation crystal affects thelight output of the crystal as a whole and the desirability of surfacetreatments has been recognized for some time (see US. Pat. No. 3,102,955and Journal of the Optical Society of America, Vol. 40, No. 11, pp.748-750, November 1950). It was found that in a finished optical crystalthe destroyed surface layer, by cutting and polishing, has a depth ofseveral microns, depending upon the hardness of the crystal, the load,and the length of time of optical working. The grinding process producesfurther destruction of the crystal surface. In addition to the materialbeing removed by fine chipping on the surface, plastic deformation takesplace in the form of flow-lines which extend into the crystal. Since theoptimum conditions for polishing can only be obtained by trial anderror, with specific pastes and creams of particular abrasives found toprovide the desired effects, known processes to provide the desireddiffusion surface on a crystal leave much to be desired because theorientation of the crystal influences the working of its surface. Theinstant process provides a way of producing a uniform, random surface ofprecisely the right texture, obviating the necessity of the trial anderror grinding and polishing by experienced hands. It has been foundthat the number of the grains per unit area, and to some extent theirsize, which are grown on the surface of the base crystal may becontrolled depending upon the wave-length of the light which thescintillation crystal surface will be required to diffuse essentiallycompletely.

In the production of known hygroscopic, ionic crystal scintillationdetectors, such as sodium iodide doped with thallium, or lithium iodidedoped with europium, it has been found that a section of a crystal cutfrom crystal ingot, when left in an atmosphere not entirely free ofmoisture, develops a surface which has a blotchy appearance. Each blotchdevelops from a cen ter of crystal growth, wherein a hydrate of thecrystal has formed. The depth of the growth effected by the localizedhydration is difficult to control, and usually results in the crystalsurfaces having to be reworked.

The manner in which sodium iodide and most other crystalline bodiesaccept scratches in machining and sanding depends both on the crystalplane of the surface and the direction in which it is scratched. Theedges of scratches consist of minute cleavage facets which account forspecular reflections and high transmissions in particular directionsbecause the cleavage facets are oriented by the base crystal structure.Grinding with coarse enough grits to leave a rough surface, even withrandom motion of the lap, shows texture and specular reflection,oriented by the base crystal. Because large crystal ingots are composedof several differently oriented optically integral components, largeground surfaces invariably show areas of different texture. The latticealignment of components does not affect light emission or transmissionwithin the body, but it affects the way in which the surface responds tooptical working and thus the behavior of light at the surface. The useof finer grits, to give smoother surfaces, still produces texture andoriented specular reflections. The instant process provides a nearlysmooth surface with uniformly random irregularities, essentially free oforiented texture and specular reflections, which enables a scintillationcrystal to perform with surprisingly high resolution unobtainable by anyother method. Crystals, other than scintillation crystals, with asurface provided by the instant invention have found utility in numerousapplications which require special surfaces.

SUMMARY OF THE INVENTION It has been discovered that, with the aid of acrystal nidus agent, another crystal different from the base crystal maybe grown non-epitaxially on said base crystal in predictably uniform butrandom manner. This is most easily done where the base crystal materialis in contact with a finely divided solid which provides a multiplicityof crystal growth centers. Thereafter, said another crystal isdestroyed, leaving the base crystal surface roughened, but nearlysmooth, with irregularities randomly arranged, or, if stable, the addedcrystal layer may be left intact, its thickness and optical prop ertiesbeing controlled by the process.

A scintillation phosphor with surprisingly good energy resolution ingamma-ray measurement is prepared by selectively treating at least onesurface of the phosphor, and preferably at least three, to impart toeach treated surface diffusion-reflection and diffusiontransmissioncharacteristics unmatched in any prior art crystal phosphor.

It has been discovered that a crystal scintillation phosphor treated asdescribed hereinabove forms a thin, roughened layer on the surface ofthe phosphor which affects diffusion and reflection of the light from aplurality of events in a crystal in such a way as to permit theconstruction of a scintillation detector with higher resolution. Theinstant process obviates the necessity of mechanically grinding thecrystal surface with a grit to provide the desirable roughened crystalsurface and the damage to scintillation efficiency near the surfacewhich this work entails.

PREFERRED EMBODIMENT OF THE INVENTION Some of the applications foroptical systems in which light diffusing surfaces are useful are asfollows:

l. to create uniform light intensity, such as is desired in sensingscintillations in a crystal to compensate for irregularities in detectorresponse;

2. to display a real image, as when focusing an optical system;

3. to compare flux density for opposing beams;

4. to accept light from wide angles; and

5. to reduce ghosts or unwanted images by treating the ends of prismsand edges of lenses or beamsplitting elements. In each of the foregoingapplications, an even-textured surface, free from oriented specularreflections, is desired, and the instant nonepitaxial growth process,with or without the subsequent destruction of new growth on the crystal,provides a desirably roughened surface which gives superior performance.

The instant invention will be described in detail with respect toroughening of at least one crystal surface of an inorganic crystalscintillation phosphor used in a gamma-ray image-type camera, though itwill be apparent that the process may be used on any crystal where theparticular physical characteristics of the instant sur face are desired.The scintillation detector is one of the oldest methods of nuclearradiation detection, but the modern scintillation detector came intobeing only after the evolution of special photomultiplier tubes for thepurpose. Surface treatments by the instant invention are contributors toa new generation of high performance detectors. Generally speaking, ascintillation detector comprises a light reflector which encases ascintillator crystal such as an optically integral, fully dense, singlecrystal, multiple crystal, or polycrystalline body of thallium-activatedsodium iodide. A light pipe channels the light from the scintillatorinto a photocathode of a photomultiplier tube, which is powered by ahigh voltage power supply. The output of the photomultiplier tube isconducted to a preamplifier, thence to a discriminator and pulse shaper,and lastly to an electronic counter or means for presentation of themeasurements in visual form. When charged particles or gamma rays arestopped by scintillators, excited states are produced which, duringtheir return to the normal states, produce discrete light flashes ofshort duration (less than 10 microseconds) or scintillations.

5 By optically coupling the scintillator with a photomultiplier tube, apulse or charge can be passed into an electronic system, making countingpossible.

Though various scintillators are available, including crystals ofinorganic and organic materials, liquids, powders, and plastics, theinstant invention is directed to crystals, particularly inorganiccrystals, in which the resolution is to be improved. This is done byproviding a thin layer of crystal growth on the surface, thereafterdestroying the growth to produce a controlledly roughened crystalsurface which appears nearly smooth. The scintillation phosphor in acamera may be a large, thallium-activated sodium iodide, Nal (Tl)crystal cut from a multiple crystal ingot. Generally, the crystalsection cut is about 13 inches in diameter and the thickness, dependingupon the energy of the radiation for which it is to be used, is in therange from about 0.5 in. to about 0.75 in. thick. Prior to treatment fornew growth, the crystal surfaces may be polished or, alternatively,

ground with a fine grit of aluminum oxide to produce a mechanicallyscratched surface. The new growth on the surface of the crystal must bethin, normally in the range from about 0.1 to l.O'microns, and 0.1 to 50microns in the longest dimension.

The controllable thickness of the new growth layer is obtained bypressing a preselected prepared crystal face against a crystal nidusagent, a mass of which is contained in a suitably large receptacle. Thecrystal nidus agent seeds the surface of the crystal in a uniformlyrandom manner, i.e., the seeding is uniform with respect to the numberof seeds for new crystal growth which are deposited on the crystalsurface, but it is random in that the orientation of grains is notrelated to the lattice of the base crystal. The crystal nidus agent maybe any finely divided solid with which is associated a transferablecompound capable of initiating nonepitaxial growth on the surface of thebase crystal.

Choice of a crystal nidus agent depends upon the base crystal material.For example, where the base crystal forms a hydrate with water, one mayuse a crystal nidus agent with water of crystallization associated withit, such as calcium sulfate (CaSO,.2H O), magnesium sulfate (MgSO .7HO), barium iodide (Bal .2- H O), sodium bromide (NaBr.2H O), and thelike. Where base crystal does not unite with water, a crystal nidusagent must be found capable of providing a moiety which can initiatenon-epitaxial growth on the base crystal. For example, where lithiumfluoride or cesium iodide is the base crystal material, boron fluorideor some of its complexes may be used with potassium fluoborate or otherpowdered solids as a crystal nidus agent. A desirable crystalnidus agentis a finely divided solid particulate form of the compound to be formednon-epitaxially on the base crystal material. For example, where sodiumiodide is the base crystal material, small crystals less than 80 Tylermesh in size, of the hydrated form of sodium iodide, namely NaI.2H O,may be used to provide the crystal nidus agent. Where lithium iodide isused, hydrated lithium iodide (LiI.3H O) may be used as the crystalnidus agent. It is immaterial whether or not the crystal nidus agent isa scintillator.

It is also required that the transferable moiety be present underprocess conditions which permit its transfer to the surface of the basecrystal, which unites with the base crystal to supply material fornonepitaxial crystalline growth, the orientation of which is unrelatedto the orientation of the grains in. the base crystal. Where thetransferable moiety is water, it may be present as water crystallizationin the crystal nidus agent provided that sufficient water is present inthis form to feed the new growth until it reaches a predeterminedthickness. Where the concentration of said moiety is insufficient toprovide the extent of new growth desired, additional amounts of themoiety may be introduced in a transferable form as required.

Crystal nidus agents which may be used successfully when associated witha transferable amount of moisture are finely divided solids such asaluminum oxide silica, magnesium oxide, magnesium fluoride, calciumcarbonate, and the like, which are available or easily comminuted to thedesired particle size by grinding, and which are easily removed afterthe desired amount of growth has been effected. The transferable moietysuch as water may be provided either in liquid or gaseous form. Carbonblack and some inorganic pigments will also function as seeds, but theyshould be avoided where the end product must have a white surface. Othercrystal nidus agents which are advantageously used are the finelydivided hydrate forms of the base crystal, For example, Nal.2H O in asize range from about 325 to about 40 Tyler mesh, in the presence ofadditional water, will grow an excellent layer of hydrated crystals onthe base crystal surface of Nal. Still other crystal nidus agents whichmay be used are macroporous and microporous alkali metal silicates andalkaline earth metal silicates, such as those silicates commonly knownas molecular sieves and which may have water, sulfur dioxide, ammonia,boron fluoride, and other non-epitaxial growth forming moietiesassociated with them in a physical manner only.

Most uniform results are obtained with a crystal nidus agent in finelydivided powder form having a size range from less than 325 Tyler mesh toabout 40 mesh. Though it is not essential that the crystal nidus agentbe a powder of uniform size, it will be found that the uniformity ofrandom growth is better when the powder is of relatively uniform size inthat its flow properties play a part in its application so that it maycontact the surface uniformly.

The crystal nidus agents may be applied to the surface of the basecrystal material by any mechanical means such as, for example, bybrushing, dusting, or packing the finely divided powdered material onthe surfaces of the crystal on which the distortion is to be generated,or by applying a suspension in an inert vehicle, such as silicone oil,mineral spirits, etc., of the appropriate crystal nidus agent.

Still another method of applying crystal nidus agents to the crystalsurface is to subject the surface to a fluidized stream carrying thecrystal nidus agents along with an appropriate amount of transferablemoiety for formation of non-epitaxial growth. The fluid conveying ofcrystal nidus agents is advantageous for application on massive crystalswhich weigh so much as to make movement of the crystals difficult. Forexample, where a crystal weighs half a ton or more, the desired amountof distortion may be generated on its surfaces by flow control of afluid carrying a predetermined amount of crystal nidus agents associatedwith a growthsustaining, transferable moiety in sufficient concentrationfor a predetermined period of time.

In each case, the base crystal material is contacted with the finelydivided solid crystal nidus agents on all surfaces on which a controlleddistortion is to be produced. In each case, the growth of new crystalmaterial generated upon the surface of the base crystal is of a compounddifferent from that of the base crystal, and grows on the surface of thebase crystal with a different lattice structure. In each case, thegrowth of the new crystalline material is unequivocally not oriented bythe orientation of the base crystal. Contact with the crystal nidusagents may be momentary. The base crystal ma terial may then be removedand set aside, awaiting sufficient formation of new crystal growth onthe surface. in general, a suitable period of time for contacting thebase crystal material with the crystal nidus agents ranges from about0.05 second to about hours, depending upon the depth of the diffusionsurface to be generated on the crystal surface and the rate of supplyfor the transferable moiety. After the desired amount of time haselapsed, the crystal nidus agent is mechanically removed from thesurface of the crystal and the crystal is treated so as todisproportionate the newly formed covering of crystal material byremoving essentially all of the chemically different growth-sustainingmoiety. Where the crystal nidus agent with the transferable moietygenerates a hydrate, after a predetermined period of time during whichthe new crystalline mate rial is grown non-epitaxially, the base crystalis placed in a very dry box, preferably under vacuum, so as to dehydratethe surface of the crystal. Selective solvents can also be found forsome combinations of materials which will remove the covering growthwithout attack upon the base crystal.

Though it is preferred to use a mass of finely divided solid crystalnidus agents, non-epitaxial growth may be effected directly by exposingthe base crystal surface to a vapor or finely divided liquid spray inthe apparent absence of the agents. As has been mentioned hereinabove,non-epitaxial growth of the base crystal hydrate is often formedaccidentally on the surface of a base crystal which is left in a humidatmosphere for too long. Characteristically, such accidental ordeliberate exposure to a humid atmosphere results in an uneven, blotchydistortion of the crystal surface, some areas being favored withnon-epitaxial growth, while others are not. The exact reason for thisbehavior is not known, but it is known that such uncontrollably unevenand non-uniform hydration on a crystal surface is detrimental. Ofcourse, if a base crystal material sensitive to hydration is leftexposed to a humid atmosphere for very long, the entire crystal surfacebecomes hydrated to such an extent as to make the crystal unusable. Forexample, in the case of the thallium-activated sodium iodide crystal,such accidental overhydration results in the formation of a yellow cruston the surface of the base crystal material and extreme amounts of waterproduce a partially liquid coating.

The non-epitaxial growth of another crystalline material on the surfaceof the base crystal occurs so long as the fugacity of thegrowth-sustaining moiety, for example, water, exceeds the partialpressure of the nonepitaxially growing crystal. For example, wheresodium iodide dihydrate is to be grown, the relative humidity mustexceed 38 per cent at 25 C. for satisfactory growth with a crystal nidusagent essentially free of transferable water of crystallization.Depending upon the particular texture desired on a specific base crystalmaterial, a mixture of various crystal nidus agents may be used.

Most interestingly, microscopic examination of the surface of a basecrystal material treated with the instant process shows certain featuresin the texture of the surface produced that differ from surfacesgenerated by scratching, grinding, or sand-blasting and a conspicuousabsence of specular reflections in specific crystal directions. Betweencrossed polarizers, on sodium iodide or lithium iodide. thenonepitaxially grown crystal material is easily recognizable, but afterdisproportionation and removal of the new growthforming moiety, theangular image remains both in the base crystal surface and in theremaining portion of the new crystalline overgrowth.

The following examples will serve more clearly to illustrate the instantinvention.

EXAMPLE 1 A machined disc of thallium activated sodium iodide crystalapproximately 13 in. in diameter and 0.5 in. thick is ground on one facein a dry atmosphere (dew point below 40 C.) with 400 grit in a lightsilicone oil using a scored glass daub or grinding tool. The disc isthen polished with an organic solvent, such as a primary alcohol or aketone, such as methanol or acetone. The clean, bright surface is thenpressed into a 0.5 in. deep bed of plaster-of-Paris of the rapid settingdental variety for about 20 minutes. The powder is then brushed away andthe plate allowed to stand for about one-half hour in a very dryatmosphere, dew point below C. After again cleaning with a jet of dryair or dusting, but not rubbing, with calcined alumina or magnesiapowder, a transparent interface coupling fluid is applied and pressedout with a glass window of larger diameter. After the coupling fluid ispolymerized, adhesively bonding crystal to glass, the excess is removed.

Exposed side and back surfaces of the crystal are then ground andsolvent-polished, as above. Plaster-of- Paris powder is added to coverthe brightly polished surfaces and allowed to remain 20 minutes. Afterremoving the powder and cleaning as above in a very dry atmosphere, theencapsulation of the crystal is completed. This involves sealing theglass plate into a heavy metal support flange that also carries a thinaluminum back cover through which gamma rays will enter. The innersurface of the aluminum back is covered with white reflector paint. Theencapsulated crystal is used as a plate for an Anger camera. In use, itis coupled to a complex light pipe which carries about 19 phototubes,the signal from which tubes are converted to an image showing thelocation of gamma rays causing scintillations within the sodium iodidecrystal.

EXAMPLE 2 A disc of thallium-activated sodium iodide is processed asdescribed in Example 1 hereinabove. The exposed sides and back face ofthe crystal are then roughened mechanically by sanding in the usualmanner, and the encapsulation is completed, as described hereinabove.The completed encapsulated crystal gives 17 per cent more output thanunits made without the nonepitaxial growth and subsequent destruction ofthat growth on the crystal surface. When this unit is installed in anAnger camera, it gives better resolution for the image produced than anyplate made by a prior art method.

EXAMPLE 3 A camera plate of thallium-activated sodium iodide is made asdescribed in Example 1 hereinabove, except that after polishing thesecond side of the crystal, it is lightly roughened by rubbing with asanding material called Scotch Brite produced by Minnesota Mining &Manufacturing Company, which consists of a fine silica grit in mattednylon fiber, more completely described in US. Pat. No. 2,958,593. Therough surface produced by hand-rubbing in such a way as to have minimumcontrast in texture between components is then covered withplaster-of-Paris, allowed to stand minutes, cleaned, dried for aboutone-half hour, and then finished as described hereinabove in Example 1.The light output of this unit is excellent and better than the unitsproduced in Examples 1 and 2 hereinabove.

EXAMPLE 4 A crystal of sodium-activated cesium iodide l in. in diameterand 1 in. thick is polished with a dampened paper tissue, then treatedin a dry atmosphere (dew point below 40 C.) with a paste consisting of10 parts potassium fluoborate powder and 2 parts boron fluoride-ethylether mixture. After a short period of time, ranging from about 1 toabout 10 minutes, the paste is removed and the surface cleaned withalumina powder, by dusting without rubbing. The crystal with a surfaceroughened by the instant process is encapsulated in the usual way, asdescribed hereinabove, with one end optically coupled to the glasswindow and the circumference and the back surface packed in aluminareflector powder. The resolution for cesium 137 gamma radiation is 8.5percent. The same crystal with a polished end coupled to the glasswindow and its other surfaces mechanically roughened according to priorart processes when encapsulated as described above gives 90 percentresolution. An improvement of 5.56 percent results with the new crystal.This improvement depends upon the ratio of photocathode area to crystalsurface area, getting substantially better as this ratio gets smaller,i.e., with large crystal surfaces for the same photocathodes.

EXAMPLE 5 A crystal of thallium-activated cesium iodide l in. indiameter and l in. thick is treated in the same manner as in Example 4hereinabove and shows a comparable improvement in resolution whencompared to the same crystal treated with prior art methods.

EXAMPLE 6 Potassium bromide crystals are treated in the same manner asthe cesium iodide crystals in the examples hereinabove. The disfiguredsurface has a texture comparable with that of crystals treated in theprevious examples.

EXAMPLE 7 A polished lithium fluoride crystal is treated with a paste ofpotassium fluoborate powder and boron fluoride-ethyl ether, as inExample 4 hereinabove, and after being in contact for about 30 minutes,is washed with a saturated solution of lithium fluoride. The crystalsurface shows a randomly oriented, disfigured texture.

EXAMPLE 8 A polished lithium fluoride crystal is coated with a thinlayer of an equal mixture of metaboric acid and potassium fluoboratepowders using as a volatile vehicle a primary alcohol, such as methanol.The dry coated crystal is then exposed to a hydrochloric acid vapor forabout one-half hour by being disposed in a glass dessicator above 35 percent hydrochloric solution. The crystal is then washed with saturatedlithium fluoride solution. The crystal surface has a randomly orienteddisfigured surface with a roughened texture essentially free of specularreflections.

EXAMPLE 9 A ground cylinder of barium fluoride crystal approximately 1in. in diameter and 0.5 in. thick is polished on one end after testingas a scintillator to cesium 137 radiation, and is then coated with amixture of 99 per cent potassium fluoborate and l per cent gypsum powderas the thin layer, using methanol as the vehicle. It is then exposed toboron fluoride gas for about an hour and then washed with watersaturated with barium fluoride. The polished surface is found to beroughened with a non-oriented texture and the specular reflections areessentially eliminated. As a scintillator in the same mountingconfiguration as before, the output was increased and resolutionimproved. Considering that the refractive index of barium fluoride is anear match to the coupling media, this is a surprising and unexpectedimprovement. Crystals of barium fluoride, calcium fluoride, strontiumfluoride and magnesium fluoride are treated as described hereinabove,giving them about 2 hours exposure to boron fluoride gas, after whichthe crystals are washed. The crystals exhibit a disfigured surfaceinstead of the original smooth surface, and the crystals are essentiallyfree of specular reflections.

EXAMPLE 10 A ground disc approximately 1 in. in diameter and 0.25 in.thick of a europium-activated calcium fluoride crystal is polished onone face; after testing for scintillation output it is treated as inExample 9 hereinabove and retested as a scintillator. The output isfound to increase from 35 percent to about 40 percent relative to Nal(Tl) as 100 percent.

We claim:

1. A process for controllably disfiguring a surface of a crystallinematerial comprising contacting said crystalline material with a finelydivided solid crystal nidus agent having a size range from about lessthan 325 Tyler mesh to about Tyler mesh for a predetermined period oftime, said crystal nidus agent being associated with a sufficient amountof a transferable moiety capable of generating non-epitaxial crystallinegrowth on the surface of the base crystal by reaction with the basecrystal, said moiety having a fugacity exceeding the partial pressure ofsaid non-epitaxial growth, and said growth occuring in a substantiallyuniform yet random manner irrespective of the crystallographicorientation of the base crystal.

2. The process of claim 1, wherein said crystal nidus agent is suspendedin an inert fluid.

3. The process of claim 1, including in addition mechanically removingsubstantially all of said crystal nidus agent after said predeterminedperiod of time.

4. The process of claim 3, including in addition dis proportionatingsaid non-epitaxial growth by removing substantially all of said moietyfrom the surface of said base crystal so as to render the surfaceessentially free of specular reflections.

5. A scintillator crystal with at least one preselected surfacecontrolledly disfigured to a preselected grain density in a uniformlyrandom manner, irrespective of the particular submicroscopic orientationof particular portions of said scintillator crystal, as a result of thedestruction of a non-epitaxial growth of crystalline material generatedby a finely divided crystal nidus agent in association with atransferable, non-epitaxial growth forming moiety, said surface beingcharacterized by a freedom of specular reflections.

6. The article of claim wherein said crystal is an inorganicscintillation phosphor.

7. The article of claim 6 wherein said inorganic scintillation phosphoris thallium-activated sodium iodide, europium-activated lithium iodide,thallium-activated cesium iodide, sodium-activated cesium iodide, orthallium-activated potassium iodide.

8. The article of claim 6 wherein said inorganic scintillation phosphoris a hygroscopic ionic crystal.

9. The article of claim 8, wherein said finely divided crystal nidusagent has water of crystallization associated therewith.

10. The article of claim 9 wherein said crystal nidus agent is calciumsulfate, magnesium sulfate, barium iodide. sodium iodide. sodiumbromide, alkali metal silicates or alkaline earth metal silicates.

11. The article of claim 6 wherein said inorganic scintillation phosphordoes not unite with water 12. The article of claim 11 wherein saidfinely divided crystal nidus agent is potassium fluoborate and thetransferable moiety is boron fluoride or complexes thereof.

13. A crystalline optical body having at least one preselected surfacecontrolledly disfigured to a preselected grain density in a uniformlyrandom manner, irrespective of the particular submicroscopic orientationof particular portions of said optical body. as a result ofnon-epitaxial growth of crystalline material generated by a finelydivided crystal nidus agent in association with a transferable.non-epitaxial growth-forming moiety. said surface being characterized bya freedom of specular reflections.

14. The crystalline body of claim 13 wherein said non-epitaxial growthof crystalline material is destroyed.

2. The process of claim 1, wherein said crystal nidus agent is suspendedin an inert fluid.
 3. The process of claim 1, including in additionmEchanically removing substantially all of said crystal nidus agentafter said predetermined period of time.
 4. The process of claim 3,including in addition disproportionating said non-epitaxial growth byremoving substantially all of said moiety from the surface of said basecrystal so as to render the surface essentially free of specularreflections.
 5. A scintillator crystal with at least one preselectedsurface controlledly disfigured to a preselected grain density in auniformly random manner, irrespective of the particular submicroscopicorientation of particular portions of said scintillator crystal, as aresult of the destruction of a non-epitaxial growth of crystallinematerial generated by a finely divided crystal nidus agent inassociation with a transferable, non-epitaxial growth forming moiety,said surface being characterized by a freedom of specular reflections.6. The article of claim 5 wherein said crystal is an inorganicscintillation phosphor.
 7. The article of claim 6 wherein said inorganicscintillation phosphor is thallium-activated sodium iodide,europium-activated lithium iodide, thallium-activated cesium iodide,sodium-activated cesium iodide, or thallium-activated potassium iodide.8. The article of claim 6 wherein said inorganic scintillation phosphoris a hygroscopic ionic crystal.
 9. The article of claim 8, wherein saidfinely divided crystal nidus agent has water of crystallizationassociated therewith.
 10. The article of claim 9 wherein said crystalnidus agent is calcium sulfate, magnesium sulfate, barium iodide, sodiumiodide, sodium bromide, alkali metal silicates or alkaline earth metalsilicates.
 11. The article of claim 6 wherein said inorganicscintillation phosphor does not unite with water.
 12. The article ofclaim 11 wherein said finely divided crystal nidus agent is potassiumfluoborate and the transferable moiety is boron fluoride or complexesthereof.
 13. A crystalline optical body having at least one preselectedsurface controlledly disfigured to a preselected grain density in auniformly random manner, irrespective of the particular submicroscopicorientation of particular portions of said optical body, as a result ofnon-epitaxial growth of crystalline material generated by a finelydivided crystal nidus agent in association with a transferable,non-epitaxial growth-forming moiety, said surface being characterized bya freedom of specular reflections.
 14. The crystalline body of claim 13wherein said non-epitaxial growth of crystalline material is destroyed.